Thank you very much for the reply and pointer to your own updated scrollbars style 'scrollable_menupopups.css'... which I had somehow managed to miss for at least the last 3 years... all whilst recommending your excellent repo's contents to others on numerous occasions!
More information on the recent changes to scrollbar styling in GitHub repo Issue #654 'scrollable_menupopups.css in V154 V155'.
Likely a facet of human nature not to look for alternative solutions to a problem that has already been solved.